Circuit for constant voltage amplifiers



Jan. 8, 1952 M L, HECHT ET AL 2,581,953

CIRCUIT FOR CONSTANT VOLTAGE MPLIFIERS Filed oct. 9. 1948 I s sheets-sheet 1 ESI JI ZL 7/ 5 3 IN VEN TOR.

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A T TORNE I/S Jan- 8, 1952 M. L. HECHT ETAL 2,581,953

CIRCUIT FOR CONSTANT VOLTAGE AMPLTFIERS Filed oct. 9, 1948 3sheetssheet 2 ATToR/VE Vs Jan. 8, 1952 M, HECHT HAL 2,581,953

CIRCUIT FOR CONSTANT VOLTAGE AMPLIFIEBS Filed 00t- 9, 1948 43 Sheets-Sheet 5 A T TOR/VE ys Patented `an. 8., 1952 CIRCUIT FOR CONSTANT voLTG-[ AMPLIFIEns Maynard-(L. Hecht, Jackson Heights, and Richar-f Sanford, Yonkers, N. Y., assignors, by mesne assignments, ,to Insl-Xi- Cprporation, Briarcliffj;r

Manor, N. Y., ascorporation" of New Jersey l Applicatien 0ctober9, 1948,` Serial No;;53;668 3 eliminas.v (Ci. rtv-ogni) i 'Ihisf-invention relates 'to a circuit fori-.a constantvolta'ge amplifier. y .i l Anf object `of this invention is r.to prov-ide aisys.- temfor'amplifyingia voltage to aconstant;prede ter-mined' higherA voltage regardless of: the: fraction of M`the vfullload` impedance Aon which .ftheizam plified Voltage is employed. I Anotherfobject ofthe invention is to provide 'a circuitk capable of maintaining a substantially constantv 4voltage :across a. varying;v load imped-v ance. .Y

Another object of the invention isto provide an electronic circuit formaintaining. `a vconstant .voltage .across a loadfimpedance by. providingvfor both positive and negativecfeed. back. A.

These and other. objects are obtained by am. plifyingi-a givenzvoltageiin an electronic amplia eriandzfeeding. back two components of therampliedvoltage l that. is impressed upon the' load. Onefof thecomponents is apositivefeed back and isfproportional to :thecurrent Ain .the load andrthe` other: componentl comprises a negative feedback; which :is #proportional kto Lthe Vpotential Adifference across .the;load.V f Y Thernovel .features characteristic of` this invention ,area set forth with- .particularity gin. Ithe appended claims;Vv The invention4 itself, "however,v both vasfto v.its organization' and its method., of

operation, -together with additional objects;- and advantages thereof, will best bei-understood. from the following description of a specic'embodi.- ment-whenread in connectionv with the accom` panying .drawings `in which: l i

Figure'l is aiblock diagram of anV ordinary-am-` plier circuit (havingv an impedance Zf in, series with theiinput).

Figure 2 is a block. diagram ofr a generalized'v circuit ofthe present invention. 5

Figure 3 a .block diagram of a modified circuit. l 4

Figurer-4 is a schematic diagram of an .actual circuit employed. i f

v Figure 5 is .az graph showing theperformanceof theicircuitof Figure 4i i The circuits Whichapplicants employ tov produce Ythefconstant voltagev amplification yare .based on certain theoretical considerations which/are best vunderstood by reference to Figure 1. Itis first-assumed'thatY it is possible to feedback.

some vcombination of voltage or current-Which will regulate the amplifier to produce a constan@y voltage acrossthe load.

In Figure 1, then, Es represents thesignalvoltvitA theaload impedance.- fZ;-,iS:,-hefimpedaiicf l transducer; as iooked into :from: the load "sfifzev Zi ,is an ,impedance i-n ser-iesmv-ithlfthe signal voltage' and the transducer across which 4there is pro"-M -f duced a voltage Ef (the/:feed back voltage) depending on the output voltage (and/or) the outputicurrent and--Zn istheload impedance..A

.In the circuit 1 represented by. Figure J1, .the total feedback voltage Ef is equal to the difference. he?lv i tween Esand Endes-,e f Y 1 f It shall fbe-,iassnmed that Egeis, by ;feedback`,.,made to change infsuch. a vvvvayl Wtih yvarying ZLthat-EL stays constant; i.

Substituting'v` Equationsv 2 arider Vin.-flilquatio"nr` 1 giyestnen-:r v

for.- the-output voltage ,PEM toremain :constan-t re- Wardless of the fluctuation of the load impedaim@Vi The-.said function can therefore be divided into two componentsor contrib'ri-tions;y Onesaof said components is proportional to the load-:current and represents a positive current feedback. 'Ihe other of said components is proportional to the voltage across the-=loadjandi tive voltageileei'ibaek representada nega If one replaces the gseneral impedance Z in Figure 1 by an amplifier tube then:

Z=Rp=plate resistance and y.=the amplification factor.

In the case of an output amplifier one will therefore have a constant output voltage if the following voltages are fed back simultaneously in series with the signal voltage:

:L The voltage represented by Equation 6 can be produced in various ways such as by providing a potentiometer to divide the output voltagepby the ration Rp/Ro. o i

` The voltage represented by Equation '1 can be produced in variousway such as by letting the load current IL flow through a resistor equal t l It willbe recognized that the developed expressions for Er| are suilciently correct only if because otherwise the feedback network would representa considerable fractionV of the total load and would have to be taken into account. The feedback network was not considered in the above derivations in so far as its effect on loading rconditions is concerned.

However, the above technique for regulation is applicable also when the change of loading caused by the feedback network is not negligiblel and formulas have been developed for designing such circuits. The fornula for the general circuit of Figure 2 taking into account finite value of the feedback primary impedances is: f

This design equation can be derived from the following consideration, y Y

If condition 9 is not satisfied then Equation 5 must be replaced by If one want to have Er. constant. then the factor Ir. gives the impedance through which the posilus tive feedback current must flow and the factor of EL gives the ratio for the negative voltage feedback network.

Equation 5b and 5c are two simultaneous equations for Zrand Zf'}. Their solution is represented by Equation 10. (Equation 5a was arrived at by using Kirchhois equations on the equivalent network.)

It is not necessary to return the feedback voltagessolely to the input of the last stagev of the amplier. They may also bek returned tov previous stages as disclosed in the block diagram of Figure 3. According 'to Figure 3 thepositive feedback is connected to the input of a driver and the negative component fedvback to the input of a pre-amplifier.

The value of y which one employs to calculate the feedback components Yin such circuits asi Figure 3 will be different as all the stages between the load and the point to wherev the feedback.- voltages are applied must be taken into account.

Figure 4 discloses a circuit employing the principle of the present invention. Three stages of amplification are shown in this figure. The input to the amplifier is -shownat 2|, 22 and the load is shown at 23. VThe input at 2| .is fed to the grid 30 of the tube 40. The input terminals 2| and 22 lare connected by a suitable resistance 49. The input connection 22 is also connected to ground as shown at 24. TA suitable resistance 50. and' condenser 60 `are connected between the; cathodev 10 of the tube. 40 and groundto furnish the proper grid bias for the tube 40 while offeringy negligible impedanceto signal frequencies.

The anodeand screen and suppressor grids of the tube 40 are connected to one side of a trans-f former 80. The other side of the transformer comprises two similar windings 8| and 82. The' outputs of windings 8| and 82 are fed to thev grids 3| and 32, respectively, of the tubes 4|.gand1 42 which together comprise the second stage of amplification, the tubes V4| and 42 being connected in Vpush-pull. VThe cathodes 1| and 12 of the tubes 4| and 42 are connected to ground over the center taps of the lament transformers and' through the resistances 5| and 52 respectively and the condensers 6| and 62,` respectively. i The anodes' of the tubes 4| and 42 are connectedto the primary of a transformer 83. 'Ihe B supply to the anodes of'tubes 40', 4| and 42 is furnished by the connection at 25. l f I' The transformer'83 has two secondary windings 84 and 85 which are connected at'one end to grids.I 33 and 34 of tubes 43 and 44, respectively. The other ends of the secondaries 84 and 85 are con-F nected to the secondary of transformer 86.

The' anodes of the push-pullampliiier formed' by tubes43 and 44 lead to thev primary of a spe-,f` cially constructed transformer 8].; The primary of this transformer comprises two portions 88and.; 89 between which are connected-(in series) --the primary 90 of a transformer 88. Resistance 51 is connected across the primary 90 of transformer., 86., The primary 90 4ofY the transformer 86 is cons-.'11

acercas;

nected at', a centen: tap to:.the;.Bf-t:j supplyzfforf thespushepull tubes 435and4-4; l p

Since; thegridsf 33..'andk 3.4.:ofrthe tubes-43 and@ 4.4- are; grounded byfway'- of twotransformercoils abias supply21i is. provided'. for:` the; cathodes 13./

portionalto the-vo1tage acrossthe load the;vo1.t;.

age from the. anode:.94 of tube 44.is fedthroughz blocking vcondenser 6.5:.and resistance. 5.3 to the flow sideof thev secondary Winding; 8| vofathe transformer 80 and similarly the voltage fromipla-tesSBf of tube 43 is fed back through blocking condenser 66 and resistance, 54 to the low. .side'fof secondary Winding 82 of transformer 80.

For the positive feedback voltage which must beproportional to; the currentv inf-.the.a1oad,. the. voltage from the secondary: off transformer. 86.l is. fed back to the low sides of the;secondarywindhings 84 and 85, respectively, of transformer 33.

The amount ofy negative feedback depends on the relative values of the resistances 53, 54, 55, and 56. Thus the amount of negative feedback to the secondary Winding v32: will.:` be determined by the ratio of resistance to the'sum'.ofresist'ances 54 andf56; Similarly theamount of negativef'eede back. to the secondary- Winding 8l.; wilfbedeter'- mined:y by the ratio-'of resistanceto the sum of resistances 53andi55..

Since the primary 90; of: the transformer 85 is in series with the load. transformer., it gives a.

voltage. across the secondary '83. which will depend on theload current. If the primary impedance of the transformer 8.6.isequal to the; factor then the transformation ratio employed must be changed. accordingly. Y v

Asthe circuit ofv Figure-v 4'qi`s shown, the nrs-t stage of amplification` isnotrapart of thepresent invention. However, if desired either the positive or negative feedback lines or both may be connected to the grid of the first tube 40, provided that only those feedback returns are used which supply the necessary sign for the feedback voltage components.

Figure shows the actual performance of the circuit of Figure 4 when:

Resistance 53=resistance 54=75,000 ohms.

Resistance 55=resistance 56=l0,000 ohms.

Resistance 51:1000 ohms.

Condenser 65=condenser 66:0.5 microfarad.

Tube 40 is the type 6SJ7 tube.

Tubes 4| and 42 are type 6B4G (class A amplier.

Tubes 43 and 44 are type GF6 (class B amplifier).

Transformer 80 is a high-fidelity transformer with a primary impedance of about 2000 ohms and a secondary impedance of 1200 ohms (grid to grid).

'Iransforr'ner` 83;* has a; primarygimpedancaof...

2000 ohms: (platefto. plate): andza secondary.` im; pedance. of .48i ohms (grid to grid-if.

Transformer 8.5. isfastransformer'with aiprimaryf;

impedance of nominally 25.0; ohms and a1 second.- ary impedance of 500 ohms and is shunted by resistance 51 which is about 1000 ohms (the shunt helps to establish a secondary voltage proportionalf with the primary current, independent-.off fre' quency.)

Taransformer 8.1 has-` a primary impedance off 6000 ohms (plate-to. plateand asecon'daryfim pe'dance of lohms.

The voltagelsupplyv 25 is +250 volts, the voltagesupply 2.6 is +350'voltsand thesupply 2T is 40.

volts.

The output resistance 2'3 is varied upf tof.1"00l ohms andV thelfoutput ofthe amplifier is adj-ustev edtoabout' llt-Watts;

cies.

load or frequency varies, and it may beduefin1 part to the fact that the impedance of the feedback circuit has not been taken into account in such a Way as to employ the theoretically most favorable values for the circuit compcnentson-tlie basis'of Equation l0. Itf is apparent however that the circuit produces a very satisfactory voltage regulation.

TheY circuit is useful' WhereverV aconstant voltage is-@to be supplied-to a varying load.

Although certain specific embodiments of this invention haveV been shown and described, it will the prior art and by the spirit of the appended claims.

We claim:

1. In a' combination, plural-strage, electronic amplification circuit adapted to provide an. output voltage which is substantially independent of the load within a certain predetermined load interval, and" having an amplifier stage having a grid input' circuit, Va subsequent amplifier stage having a grid input circuitV andan output coupling means beyond the said subsequent amplifier stage, the improved construction which comprises, a. negative feedback circuit including a resistive component connected between said output coupling means and the grid circuit of said rst named amplifier, and a positive feedback circuit including a resistive component oonnected between said output coupling means and the grid circuit of said subsequent amplifier, the impedance of said negative feedback circuitl having a value so as to supply a voltage to the grid circuit equal to where EL equals the voltage desired across the load, Z is the output impedance of the amplifler Z0 is the total resulting load impedance for which the voltage desired across the load is obtained without feedback and ,L is the equivalent inplication factor f the amplifier between the output and the point where the feedback is applied, the impedance of said positive feedback circuit having a value so as to supply a voltage to the grid circuit equal to -Z- X I L y where IL is the load current Z is the output impedance of the amplifier and p.' is the amplification factor of the amplifier between the output and the point where the feedback is applied.

2. In a combination, plural-stage, electronc amplification circuit adapted to provide an output voltage which is substantially independent of the load within a certain predetermined load interval, and having an amplifier stage having a grid input circuit, a subsequent ampliiierstage having a grid input circuit, and an output transformer beyond the said subsequent amplifier stage, the improved construction which comprises, a negative feedback circuit including a resistive component connected between said output transformer and the grid circuit of said rst named amplifier, and a positive feedback circuit including a resistive component connected between said output transformer and the grid circuit of said subsequent amplier, the impedance of said negative feedback circuit having a value so as to supply a voltage to the grid circuit equal to #ZOXEL where EL equals the Voltage desired across the load, Z is the output impedance of the amplifier Zo is the total resulting load impedance for which the voltage desired across the load is obtained without feedback and ,u is equivalent amplication factor of the amplier between the output and the point where the feedback is applied, the impedance of said vpositive feedback circuit having a value so as to suppl; a voltage to the grid circuit equal to IXIL where IL is the load current Z is the output irnpedance of the amplier and u is the amplification factor of the amplifier between the output and the point where the feedback is applied.

3. In a combination, plural-stage, electronic amplification circuit adapted to provide an output voltage which is substantially independent of the load within a certain predetermined load interval, and having an aimpiier stage hav-f ing a grid input circuit, and an output transformer beyond the said subsequent amplifier stage, the improved construction which comprises, a negative feedback circuit including a resistive component connected betweeri said output transformer and the grid circuit of said first named amplifier, and a positive feedback circuit including a resistive component connected between said output transformer and the grid circuit of said subsequent amplifier, said subsequent amplifier stage being a push-pull amplifier working into said output transformer, the impedance of said negative feedback circuit having a value so as to supply a voltage to the grid circuit equal to f l (Z',+Z(1 +2 )XEL) L- #Z0 and the impedance of said positive feedback circuit having a value so as to supply a voltage to the grid circuit equal to MAYNARD L. HECHT. RICHARD SANFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,655,557 Mathes 1 Jan. 10, 1928 1,658,346 Mathes Feb. 7, 1928 2,173,427 Scott Sept. 19, 1939 2,220,770 Mayer Nov. 5, 1940 2,245,598 Llewellyn June 17, 1941 2,270,012 Shepard Jan. 13, 1942 

